The invention relates to a high-pressure discharge lamp having a quartz glass discharge vessel which encloses a discharge space with a filling comprising at least a rare gas and Hg, in which a first electrode and a second electrode are present between which a discharge extends during lamp operation, and having a first seal incorporating an electric conductor which connects the first electrode to a metal wire projecting from the first seal to the exterior, said first seal having a first and a second gastight portion between which a gas-filled cavity is present.
A lamp of the type described is known from WO 97/48116. The known lamp is suitable for operation in air, i.e. free from an outer envelope. For lamps intended for an accurate formation of a beam by means of an optical system, this is an important advantageous aspect. Particularly for applications such as, for example, in projectors, and motor vehicle headlamps, the avoidance of optical disturbances caused by an outer envelope plays an important role. It is important that the temperature of the electric conductor has a relatively low value at the area where it is exposed to air, in order that a rapid oxidation of the conductor is prevented. In the known lamp, this is realized by elongating the seal by means of a second gastight portion spaced apart from the first gastight portion, between which a nitrogen-filled cavity is present. In this description and the claims, quartz glass is understood to mean a glass having an SiO2 content of at least 95%.
In high-pressure discharge lamps, ignition delay often occurs in practice when igniting the lamp. The risk of an ignition delay strongly increases when the lamp has been in the dark for some time. The occurrence of ignition delay is a great drawback and, under circumstances, may lead to dangerous situations, for example, when using a high-pressure lamp as a motor vehicle headlamp.
It is an object of the invention to provide a measure for obtaining a lamp as described in the opening paragraph, in which the drawback described above is obviated.
According to the invention, a high-pressure discharge lamp of the type described in the opening paragraph is characterized in that the cavity comprises a further rare gas or a gaseous constituent of the filling or a combination of both, and in that the first seal has a first external antenna at the area of the cavity.
The lamp according to the invention has the advantage that, without detracting from the stability of the first seal obtained, the available cavity constitutes a start-promoting means as a source of UV radiation when applying an electric voltage across the cavity. The UV radiation source is referred to as UV enhancer.
Although it is preferred to position the UV enhancer as closely as possible to the discharge vessel, the provision of a cavity suitable as a UV enhancer in the first gastight portion of the first seal affects the mechanical strength of the first seal of the discharge vessel and is therefore not applicable in practice. For optimum beam properties of an optical system, minimal dimensions of the light source are desired, which is realized by choosing the distance between the first and the second electrode as small as possible. A practical result is that high to very high operating pressures occur in the discharge vessels of such lamps. Consequently, this imposes very strict requirements on the gastight seals of the discharge vessel. Positioning the UV enhancer at a larger distance from the discharge vessel, namely behind the first gastight portion surprisingly does not appear to detract noticeably from the ignition-promoting effect of the UV enhancer.
In an advantageous embodiment of the lamp according to the invention, the electric conductor is a foil which extends across a length of the first gastight portion, the cavity and the second gastight portion. On the one hand, this is a considerable simplification of the seal construction and its manufacture and, on the other hand, it has the important advantage that a strong concentration of an electric field is produced at the edges of the foil as soon as a voltage is applied to the conductor. This enhances breakdown in the UV enhancer.
The first seal in the lamp according to the invention is preferably a collapsed seal. This has the advantage that the glass has adhered to the electric conductor by means of flowing at the area of the first seal so that the gastight seal is free from internal stress to a considerable extent.
In a preferred embodiment of the lamp according to the invention, the first external antenna is electrically connected to the second electrode. A passive serial antenna combining a simple structure with a very reliable operation is thus realized.
In a further advantageous embodiment of the lamp according to the invention, the seal is connected to the discharge vessel at the area of a neck, and a second external antenna is present at the area of the neck. Surprisingly, the positioning of the second external antenna provides the possibility of considerably reducing the possible ignition delay upon re-ignition of the lamp directly after it has been extinguished, which effect is further referred to as hot restrike. For reasons of a simple construction, the second external antenna is preferably connected electrically to the first external antenna. The first and the second external antenna may constitute an active system in which ignition voltage pulses are exclusively applied to the antennas. In the case of a passive system, the antennas are connected to one of the electrodes, preferably the second electrode. Both in the case of an active system and in the case of a passive system, the first external antenna enhances a substantially immediate breakdown in the UV enhancer as soon as an ignition voltage pulse is applied, and the second external antenna contributes to the realization of a minimal delay upon hot restrike of the lamp. The cavity already appears to function eminently as a UV enhancer if it contains only one rare gas. This may be the rare gas which forms part of the filling of the discharge vessel. However, it is alternatively possible for the cavity to contain another rare gas, referred to as further rare gas. Preferably, the gaseous constituent of the filling in the cavity comprises mercury vapor. This has the advantage that relatively much UV radiation is generated by the UV enhancer, which particularly contributes to a rapid and reliable hot ignition. A further advantage of the lamp according to the invention is that no separate mercury dosage appears to be necessary. This is easily realizable by making the first seal after the discharge vessel has been provided with its filling. For the purpose of electrical connection of the second electrode, the lamp is provided with a second seal for feedthrough of an electric conductor to the second electrode. For reasons of an efficient production of the lamp according to the invention, this second seal has the same construction as the first seal.
The first and second external antennas can be formed by separate pieces of wire, for example, wound around the first seal. In an alternative embodiment, a single wire is wound around the first seal in such a way that both the first and the second antenna are formed. In an advantageous embodiment, each antenna is formed by a resilient clamp body which clips partly around the seal.
The first and second external antennas can be formed by separate pieces of wire, for example, wound around the first seal. In an alternative embodiment, a single wire is wound around the first seal in such a way that both the first and the second antenna are formed. In an advantageous embodiment, each antenna is formed by a resilient clamp body which clips partly around the seal.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.